A. Field of the Invention
This invention relates generally to bar code readers, and more particularly, to bar code readers utilizing logarithmic converters for providing to a microprocessor signals indicative of the logarithm of the width of the bars and spaces of a bar code record.
B. Description of the Prior Art
Bar code record readers are known. One such bar code reader is described in related U.S. Pat. Nos. 3,784,792 and 3,886,521 issued to Bruce W. Dobras, assigned to the same assignee as the assignee of the present invention, and incorporated herein by reference.
The system disclosed in the referenced patents utilizes a scanner for scanning a coded record. The scanner controls a plurality of counters that accumulate counts proportional to the widths of the individual bars and spaces being scanned. A determination of whether a particular bar or space is wide or narrow is made by taking the ratio of the accumulated counts corresponding to two of the scanned bars or spaces, and comparing the resultant ratio with a pair of constants. If the ratio thus obtained is larger than a first constant, the first scanned bar or space is considered to be wider than the second scanned bar or space. If the ratio is less than a second constant, then the first scanned bar or space is considered to be narrower than the second scanned bar or space. If the value of the ratio falls between the values of the first and second constants, the bars or spaces are considered to have equal widths.
While systems of the type described in the above referenced patent operate satisfactorily, the advent of microprocessors has made it possible to reduce the complexity while increasing the flexibility and versatility of such systems. However, the nature of the bar code scanning process is such that the ratio of numbers representative of the widths of the two bars or spaces must rapidly be obtained with an accuracy beyond the capability of the present day integrated circuit microprocessors. Consequently, multiple microprocessors must be utilized, or the width data must be compressed prior to taking the ratio.
Two such data compression systems are disclosed in U.S. Pat. Nos. 3,854,036 and 3,887,792. The system described in the U.S. Pat. No. 3,854,036 utilizes a read-only memory for converting the width representative counts to logarithmic representations of the widths. These logarithmic representations may readily be subtracted from each other by a microprocessor in order to determine the ratios of the widths of the various individual bars and spaces. Such a logarithmic data compression system permits a microprocessor to be used, but no attempt has been made to select a base for the logarithmic conversion that provides optimal resolution for a given read-only memory and microprocessor. The system described in the U.S. Pat. No. 3,887,792 utilizes a digital log-time generator to compress the data, but again no attempt is made to optimize the accuracy of the system by selecting an appropriate logarithmic base. Consequently, either accuracy must be sacrificed, or the decoding system must be made more complex than necessary.